1. Field of the Invention
This invention relates to power failure indicating circuits for power supplies. More particularly, the invention relates to such a circuit for a pulse width modulated power supply.
2. Description of the Prior Art
Electronic test instruments such as oscilloscopes often use logic circuits such as microprocessors in their operation. These circuits store and transmit information to control the instrument and record data obtained by the instrument. It is desirable to preserve much of this information and recorded data when power to the instrument is shut off or is unexpectedly interrupted, such as occurs during a brownout or transient overload conditions. These conditions can arise in a plant, for example, from the continuous switching on and off of central heating and air conditioning systems.
Prior designs for monitoring the operation of a pulse width modulated power supply of an instrument have typically sensed the level of the AC line input voltage into the power supply. When the input voltage level falls below a designated threshold level, a warning signal is transmitted to the logic circuits to allow them to store their information in anticipation of the line voltage falling further and causing a loss of the digital information. The warning signal, however, is often premature because the power supply voltage is maintained temporarily above the falling line voltage by storage capacitors within the supply. Many of the transient conditions of the line voltage are so brief that the storage capacitors are able to maintain a sufficient voltage to ride out the interruption. Monitoring the AC line voltage thus can cause unnecessary shutdowns of the instrument, resulting in costly delay and potential information loss.
Those skilled in the art have recognized this drawback of prior designs but have been unable to avoid it because of the required presence of a galvanic isolation barrier between the pulse width modulated power supply and the internal instrument circuitry to minimize danger to the instrument operator. The DC voltage on the capacitors cannot be sensed directly by internal circuitry because a DC signal cannot cross the galvanic barrier. One technique that avoids the barrier is to optocouple signals from the power supply side of the barrier to the internal circuitry side. Optocoupling circuits, however, are quite expensive and lack needed accuracy and thus impractical for most instruments.